The present invention relates to techniques for controlling the level of a signal. More specifically, the present invention provides methods and apparatus for controlling the level of an audio signal.
Virtually all audio amplification systems require a means of controlling the overall gain of the signal path. Such gain control enables the system engineer to optimize signal levels to fit the dynamic range of the system and allows end-users to adjust the loudness or volume of the amplified sound to suite comfort levels or taste. Volume control can be achieved by means including variable resistive elements, e.g., potentiometers, in the analog audio path, variable gain amplifiers (VGAs) in the analog audio path, and multiplication of digitized audio by a digital volume control word.
In multi-channel systems such as 2-channel conventional stereo or 4-6 channel surround audio systems, potentiometers ("pots") are often ganged on a common rotational shaft such that all channels receive roughly the same degree of gain control. Alternatively, multiple VGAs can be used for a plurality of channels, each receiving the same gain control signal as all others, in order to achieve a uniform overall gain setting. In the digital domain, each audio channel receives the appropriate digital gain parameter value.
In the case where individual channel gains need to have a relative offset but still track together globally, such as in left/right balance (pan) or interchannel trimming, additional potentiometers can be added to the pot-based system in series with the master volume control or a slip clutch mechanism can be used to allow individual pot adjustability in the ganged pot configuration. Also, per-channel offsets can be added to the global gain control signal in VGA-based systems, and numerical offsets can be similarly implemented between the channel gain parameters in the digital system.
Audio volume control circuits must typically satisfy a broad range of requirements. For example, such circuits should have a logarithmic transfer function to match the nature of human loudness perception. A logarithmic transfer function is achieved in pot-based systems by the use of "audio taper" pots which have a logarithmic variation in their resistance as a function of shaft rotation. This works well in most applications, with the disadvantage that it can be difficult to inexpensively manufacture pots which can precisely match one another for ganged use. As a result, in modestly priced audio systems differences between channel volumes at low volume settings are often easily perceived. In VGA-based systems, the logarithmic volume variation is implemented by mapping of the control signal from linear input (e.g., a voltage from a potentiometer or DAC) into the appropriate logarithmic form. Alternatively, the control voltage can be derived from an audio taper pot. In digital volume control systems, the logarithmic volume steps can be achieved with a mapping function, e.g., from a simple look-up table.
Audio volume control circuits must also exhibit low noise. Potentiometers, since they are passive devices, contribute no active noise to the signal path, but can degrade audio signals with resistor thermal noise, and discontinuity noise from, for example, a dirty wiper contact. In some cases pots also allow electromagnetic interference to enter the audio path due to inadequate shielding. VGAs are active devices akin to operational amplifiers and therefore inherently contribute some degree of noise. This noise can be minimized with adequate design techniques, but this comes at additional cost due to larger signal handling transistors or increased bias current in gain stages. In digital volume control systems, if implemented exclusively in the digital domain, the system noise is governed by the bit resolution of the system following the volume control block. For example, if one has a 16-bit digital audio system with digital-only volume control, this implies that full output loudness correlates to activity in all 16 bits at the DAC which drives the power amplifier. If one then sets the volume to be 1/4 of the maximum available dynamic range, i.e., a volume reduction of 2 bits, one is left only using 14 bits at the DAC--a distinct cut in resolution. Also, noise and distortion products contributed by the DAC and any subsequent EQ are not attenuated when volume is reduced. The noise floor can thus become apparent at even normal volume settings. It is for this reason that all-digital volume controls are usually not used, and instead a hybrid of pre-DAC digital control and post-DAC analog control is used if the system is to include any volume control in the digital domain.
Audio volume control circuits must also exhibit low distortion. Passive, potentiometer-based volume control systems are essentially distortionless. The exception to this might be very slight voltage-based resistor value dependencies. VGAs are active amplification devices and are therefore subject to the usual set of non-ideal characteristics inherent in any active gain block which can contribute to overall distortion. All-digital volume controls could potentially suffer from distortion due to truncation or rounding errors in the multiplication process.
Audio volume control circuits must also exhibit transition smoothness. Changes from one gain setting to another should be done on a gradual scale to prevent the introduction of audible artifacts into the audio. After all, gain control is actually the multiplication of the hi fidelity audio signal(s) with a quasi-static control signal, and any aberrant behavior in the control signal will produce modulation products in the audio signal. The "trick" is to keep all variations in the gain control signal sufficiently gradual, e.g., with frequency components below 10 Hz, and/or small in amplitude, such that the modulation products remain unnoticeable or unobtrusive. Hand-operated potentiometers inherently provide a relatively slow and smooth transition from one gain setting to the next by virtue of the limited rate at which the human operator can turn the control knob. If, however, the potentiometer(s) is (are) operated by a stepper motor (as in the case of remote control) there is more risk that the individual stair steps of volume change will be noticeable, depending on the servo-stepper design. In VGA systems, the gain control signal must be made to ramp smoothly between gain settings even if the command to change is a step function. This is easily achieved if the signal is derived from a continuous source such as a pot, but requires more care if the control signal is derived from a more coarse source such as a DAC. In digital volume control systems, smaller, intermediate gain steps can be added between allowed volume setting increments, permitting the control algorithm to more closely approximate a smooth ramp during volume changes. For example, if the volume control is only allowed to sit on integer dB positions such as 0 dB, -1 dB, -2 dB, and so forth, it could be made to micro-step between these steps in 1/4 dB increments to reduce the audibility of the 1 dB steps.
Audio volume control circuits must also enable precise tracking between channels in multi-channel systems. That is, where two or more channels are involved, it is usually desirable that the volume control function for each channel closely match that of the others throughout its entire gain control range. Offsets can be intentionally introduced between channel gains for purposes of left/right balance or front/rear inter-channel trimming, but once these offsets are determined it is again necessary for all channels to faithfully track the master gain control signal in proper proportion. In potentiometer-based systems, as mentioned, pots are often physically ganged onto a single, rotating shaft to achieve tracking. Left/right balance or inter-channel trimming can be achieved either by the use of concentric shafts controlling the individual pots with a slip-clutch mechanism to achieve a ganging function with relative adjustability, or by the use of additional pots in series with each channel for balance/trimming. The former method has the disadvantage of mechanical complexity and does not function well at very low or very high volume settings because one offset channel will hit the the extreme stop point before the other(s). The latter method has the disadvantage of requiring more pots. In a VGA system, it becomes necessary to use VGAs which have closely matched gain control functions. In the digital domain, precise tracking is easily implemented given the exacting nature of the digital calculations involved.
Audio volume control circuits must also exhibit a wide dynamic range. A typical operating range for a volume control system is 80-100 dB gain variation. Ideally, a volume control system would be able to pass all 96 dB of the dynamic range contained in a commercial CD recording even when at its minimum gain setting, implying a signal path dynamic range of nearly 196 dB. Of course at minimum gain setting (before muting altogether) the volume is so low that much or most the 96 dB of the CD recording is lost to human hearing, so the 196 dB figure is unnecessary in practical terms. Nonetheless, it will be understood that the dynamic range required of a good volume control circuit is well in excess of the audio program material which it passes. For potentiometer-based volume controls this is generally not a problem, since the passive nature of the resistor usually adds little noise (if low in resistive value and appropriate material is used) and virtually no distortion. Low distortion is more of a challenge for the VGA system because of the active nature of the gain-controlling amplifier circuitry. The VGA will add some limited degree of distortion which usually increases with lower gain settings. As discussed above, it will also add some degree of noise. In an all-digital volume control system the output of the volume control block would have to be of significantly more bits than 16 to achieve the type of dynamic range which is desirable. For example, if the volume control word is 8-bits and the audio 16-bits, the resulting gain-controlled output would be 24-bits. This 24-bit word would have to pass through an equivalent 24-bit DAC before being fed to the power amplifier. And, as will be understood, a DAC of that resolution is prohibitively expensive. Some would even debate whether true 24-bit DACs are actually realizable with today's technology.
Digital control is also desirable for audio volume control circuits. With the advent of digitally operated audio equipment such as remote-controlled home stereo receivers, televisions, and other home entertainment systems, the need arose for the digitally operated volume control. Here, a relatively low resolution binary control word of perhaps 6 to 8 bits (i.e., 64 to 256 levels) is used to select gain levels over the operating range of the volume control in a logarithmic scale. This can be applied in potentiometer-based volume control systems with the use of a remotely operated stepper motor actuating the rotary shaft of the pot(s). The added cost and undesirable complexity of such a setup is relatively easy to imagine. In VGA-based systems, a DAC can be used to receive control commands from some remote source and convert them into gain control signal(s) as appropriate. In all-digital systems, the volume control word (appropriately mapped into logarithmic form) is simply multiplied by the audio signal to produce a volume-controlled result.
Reliability is, of course, a desirable characteristic of volume control circuits. A potentiometer is an electromechanical item which is subject to the deleterious effects of heat, moisture, dust, corrosion, vibration, and simple wear and tear. In many consumer electronic devices, it is among the first subsystems of the product which develop problems, usually due to dirt or dust contamination. In addition, if a motor of some sort is used to actuate the potentiometer digitally or remotely, the electromechanical complexity increases and reliability correlatively decreases. VGAs and digital volume control implementations are built mostly or completely from discrete or integrated semiconductors with perhaps some passive components, and exhibit the high degree of reliability associated with such components.
It is also desirable that audio volume control circuits are both easy to implement and easy to use. A potentiometer or even a gang of two or more is very easily incorporated into an audio system as well as easily operated. This is less true if a stepper motor actuator is employed for digital or remote control. A VGA system is relatively easily designed by an experienced circuit designer, but can be somewhat complex. Digital implementation is fairly straightforward to an experienced digital ASIC designer but adds complexity to the DAC design.
System simplification and cost reduction by means of circuit integration is also essential in most audio marketplace sectors. A single, ganged, or servo-driven potentiometer simply cannot be integrated into an IC. The VGA approach can be integrated, but makes for a fairly complex analog IC function if high performance is targeted. The digital approach, of course, integrates naturally.
Finally, low cost is desirable for audio volume control circuits. Single or double-gang potentiometers are inexpensive. Concentric slip-clutch potentiometers are more costly, and pot combinations which are sealed against dust and have good inter-pot tracking are even more expensive. When a servo operation mechanism is added, the cost goes up dramatically. VGAs are not particularly expensive, but are more costly than op amps of similar performance ratings simply because they are sold in lower quantities. A digital volume control implementation has only a small incremental cost associated with it because it comprises only a modest number of gates with respect to the usual complexity of a digital audio ASIC of which it would be a part.
It is clear from the above discussion that each of the currently available techniques for implementing volume control in audio systems has its disadvantages with regard to at least some of the desirable characteristics of volume control circuitry. It is therefore desirable to provide volume control technology which exhibits all of these characteristics as well or better than the technologies discussed above.